determining alkalinity via pH and co2*

[karimwassef]

I like the idea of measuring in-water CO2 as well as the air being injected.

It would help optimize my air injection. I'm currently running on maximum pressure ~30ft through two penductors with a massive vent fan to pressurize the external air through an 8" duct... overkill! But I know that I'm getting gallons of fresh air heavily aerating.

[dartier]

I read the thread on the planted forum, both the one I found and the one that orcafood posted. They were by the same person. In both cases he was able to measure the amount of CO2 in his FW tank using the inexpensive IAQ CO2 sensor.

The 2 challenges he had was keeping the sensor dry, he was mounting it inside the housing with a silicone membrane and putting it below the water surface.

My suggestion would be to use the remote sensing cap that they offer and use an aqualifter pump to setup a closed loop through the sensing head. The pump offered from the vendor is quite pricey. For the sensing head, a 1/2" PVC tee, with the inline legs sealed with a 1/2" to 1/4" NPT adapter so that a John Guest fitting can be screwed in for each side of the loop.

For the 3rd leg of the tee, the membrane needs to seal this portion. I am still thinking about how to do this and make it maintainable. I am thinking a union that can be tightened on with the membrane trapped between the O-ring and the union face may be all that is needed. The breather bags were slow to pass the CO2 and reach equilibrium, but for our needs, that may be fine.

Lastly, they found the sensor was much quicker to update the lower below the water surface it was positioned. So for safety it would be nice to be able to detect a leak for water incursion and have the pump not pump it through the sensor. Still thinking about how to do this without impacting the internal air volume in a significant way.

Dennis

[orcafood]

Originally Posted by karimwassef

I like the idea of measuring in-water CO2 as well as the air being injected.

It would help optimize my air injection. I'm currently running on maximum pressure ~30ft through two penductors with a massive vent fan to pressurize the external air through an 8" duct... overkill! But I know that I'm getting gallons of fresh air heavily aerating.

It would be very interesting to see how stable the levels of dissolved CO2 are in your setup

[karimwassef]

I would compare the air CO2 to the water CO2 at different injection pressures over time...

The high pressure injection reduces the time for the air CO2 to impact the water CO2.

The water CO2 is not benign. It has its own forcing function as the tank actively consumes CO2... so the injection both adds and removes CO2 to keep the air and water tight.

[bertoni]

Is there any documentation on the type of silcone that he used? That's another interesting idea.

[dartier]

Originally Posted by bertoni

Is there any documentation on the type of silcone that he used? That's another interesting idea.

I am not sure if this is the product he used, or he DIY'd one, but he did link this site as a possible supplier: http://www.sspinc.com/products/Thin-...4_category.htm

At one point he was talking about making his own between 2 pieces of saran wrap with a rolling pin, lol.

He mentions that the site above has 12" x 12" sheets that can be purchased.

Dennis

[karimwassef]

The silicone diffusion rate would need to be factored in.

I'm interested in time ... so this is a big deal.

[bertoni]

That looks like an interesting approach. I agree that we'd need to know the velocity of carbon dioxide across the membrane. It'd be an interesting experiment if the cost is tolerable.

[karimwassef]

I'm trying to gauge the rate of diffusion of air CO2 into aqueous CO2 and need to account for the rate of diffusion of CO2 through the membrane!

Don't like measuring time with a time error... but maybe it can be corrected for enough.

[orcafood]

I think a pipe sealed from the top in an overflow (low flow area) extended all the way to the bottom having no cap and some sort of inline fan like an aqualifter to keep the air moving through the top of the pipe to the sensor and back near the bottom so it is always full of air? Keep the sensor far from the water and bring air, which is under decent pressure from the top-capped bottom-open tube submerged. Then pack a bunch of desiccant into an inline filter before the sensor.

[dartier]

What range do you think we should be planning to support? The vendor linked with the IAQ sensors offers a variety of ranges (and price points), with the most affordable supporting 0-10,000 ppm while ones that support 0-100,000 ppm are 3 x the price (and 1/3 as accurate 30 ppm | 100 ppm).

Is 10,000 ppm a reasonable limit to expect that may diffuse out of the water into the headspace of the sensing head?

Dennis

[orcafood]

There is around 400 ppm in the air outside, closer to .5 ppm dissolves into the tank water as co2, a headspace should give a depressed readout compared with the outside co2, low range is best.

[karimwassef]

300-900 is what I've found meaningful for my reef

[dartier]

Here is a new device that is coming onto the market that would be a lot easier to use. no pricing just yet.

https://www.atlas-scientific.com/pro...s/ezo-co2.html

However in a thread on R<2>R between RHF and a couple of Alkalinity tester people, they indicated that to have a hope of being able to accurately calculate Alkalinity based on PH and CO2, that you would need an accuracy of 2-3 ppm for the CO2 reading. Which none of the reasonable priced detectors seem to be able to do (including the one I just posted).

Dennis

[karimwassef]

Can you spell out the thread after the clay-boa?
Haven't seen RHF on here for a while

[dartier]

Originally Posted by karimwassef

Can you spell out the thread after the clay-boa?
Haven't seen RHF on here for a while

Randy does not post here much anymore as far as I can tell. He handles the chemistry forum on R<2>R now. If you search for "KH Guardian(KHG) Alk calculation" over there you will find it.

Kirill is also active in that thread. It was his DIY alk monitor (on here) that piqued my interest so many years ago to start thinking about building my own version.

Pretty silly that I have to search for ways to encode a short form to get it past the censors on this board. Unbelievable actually ...

Dennis

[orcafood]

Though the resolution is low on the CO2 sensors the CO2 levels in the water should not change too quickly. I'm thinking that fact can be abused in combination with a robust outlier rejection (RLS) system to get a more stable approximation. If we combine hundreds of data points over the course of a few minutes a sizable distribution can be generated.

Another option would be boosting the CO2 signal somehow. Perhaps a dye could be used, reactive to CO2, to enable detection via visible light. I'm thinking a dosimeter with a color based detector could work for a while.

Building a more accurate sensor might be feasible as well, they don't seem very complicated. IR led of the proper nm, maybe a laser diode, small tube with air flow and a very accurate IR detector protected with a band pass filter.

wiki seems to disagree though haha, I don't see why a more powerful IR source couldn't get higher resolution.

"The best of these have sensitivities of 20–50 PPM.[1]"

FTIR lists a detection limit of 50 ppb for CO2 in air. Why would it have such a lower detection limit over NDIR?

-- The interferometer used to sweep the spectrum in the FTIR is why the detection limit is lower

[bertoni]

If the error follows some known distribution, then using more measurements should help with accuracy. If there's a deterministic bias of some sort, that won't work, though. It would be interesting to see what kind of data these meters produce.

[orcafood]

My question is how do they not have FTIR setups for narrow bandwidths. Must be cheaper to produce an FTIR that can't sweep the full spectrum. My principles of instrumental analysis book is saying that multiplexing the signal is the reason for the FTIR gaining so much sensitivity over the NDIR. Another reason is due to the Jaquinot advantage or throughput advantage. Fewer optical elements and no slits allows for a much stronger signal.

Ahhh:

"decreasing the width of the resolution element has the unforunate effect of decreasing the s/n ratio due to needing narrower slits. For infrared detectors, the reduction in signal strength is not accompanied with a corresponding decrease in detector noise.

The multiplex advantage is described as overlaying many FT spectra and increases the s/n ratio of IR over Vis by a factor of 40. It is unusable at near-IR/Vis due to shot noise and flicker noise which both increase with radiant light source power.

[orcafood]

http://www.uicinc.com/blog/compariso...ection-method/

"Detection Range

Since a single CO2 molecule may be “counted” several times as it passes through the cell, the NDIR detector can be very effective at conducting low-level carbon analyses. Longer cell lengths allow for lower level analysis, often down to the low part-per-billion (ppb) range."

[orcafood]

It seems there are a few types of NDIR. One type of NDIR called gas correlation analysis is right on the money with what is needed here, it uses a reference gas and differential pressure to transduce the signal with a capacitor. They have a resolution around 1-2 ppm with a folded path of 3 meters. A longer folded path would get even more accurate of results but doesn't seem necessary.

I bet an old one could be picked up off of ebay still functioning for a few hundred dollars and interface it through an analogue pin on a PIC.

[jrhupp]

Originally Posted by orcafood

It seems there are a few types of NDIR. One type of NDIR called gas correlation analysis is right on the money with what is needed here, it uses a reference gas and differential pressure to transduce the signal with a capacitor. They have a resolution around 1-2 ppm with a folded path of 3 meters. A longer folded path would get even more accurate of results but doesn't seem necessary.

I bet an old one could be picked up off of ebay still functioning for a few hundred dollars and interface it through an analogue pin on a PIC.

I think you want to avoid this type of sensor; there is a reason they are not really used anymore. They have strong temperature dependencies and are sensitive to vibrations.

It would be much better to find a sensor that does the measurement purely optically.

I haven't done anything with the little NDIR sensors targeted at HVAC systems, but I think they hold good potential for this application. My guess is they will be a fail out of the box, but that with some effort/modification they may perform pretty well. At this point though, that is just speculation.

[orcafood]

Ah good point with the vibrations/ temperature issues, I just noticed they are very accurate when done properly.

I think a longer path length for the NDIR would help tremendously with the resolution. Also I bet a beam chopper setup could help reduce some of that noise. If I remember keeping the detector cooler might also lower the noise.

I think the problem is why have such a small NDIR? The available filtered NDIR photometers I have seen seem to be really small like they are trying to make them portable or something. Really it is just a box with mirrors setup to increase the path length. The IR source is just nichrome wire, dirt cheap, band pass filter isn't too bad then lastly a decent pyroelectric transducer which is gona be the pricey part. I'm seeing that thermopiles can be used for mid-IR, shouldn't be too expensive.

Someone must make a larger, higher resolution CO2 monitor...


I have begun to collect broken cameras for optics haha

[jrhupp]

Originally Posted by orcafood

Ah good point with the vibrations/ temperature issues, I just noticed they are very accurate when done properly.

I think a longer path length for the NDIR would help tremendously with the resolution. Also I bet a beam chopper setup could help reduce some of that noise. If I remember keeping the detector cooler might also lower the noise.

I think the problem is why have such a small NDIR? The available filtered NDIR photometers I have seen seem to be really small like they are trying to make them portable or something. Really it is just a box with mirrors setup to increase the path length. The IR source is just nichrome wire, dirt cheap, band pass filter isn't too bad then lastly a decent pyroelectric transducer which is gona be the pricey part.

Someone must make a larger, higher resolution CO2 monitor...


I have begun to collect broken cameras for optics haha

For a broadband measurement (which is what I think we want), the short path lengths are perfectly reasonable. It is no issue to get resolutions on the order of a tens of ppb with a path length of 10 or 15 cm.

To get a significantly longer path length requires a much higher power IR source. Now days that general means a near IR laser (costly) or mid IR laser (supper costly) which is just a ridiculous approach in this application.

Agreed that the IR source should be cheap. The real cost in the source will be the reflectors and collimating optics.

The detector should be pretty cheap too. Nothing too special there for what we are discussing.

It will be the optical filters that will be the biggest component cost and most trouble to select. You need to pick a region of strong absorption that is free from spectral interference from other atmospheric gases and chose a filter that covers a sufficient portion of this region so that you capture it across a range of temperatures and pressures and with sufficiently sharp cut offs on the ends. This is a far from trivial excise and one off filters will likely be a couple hundred bucks a pop.

As to the camera optics, keep in mind that you are dealing with wavelengths far beyond visible. So just because you can see through it doesn't mean IR can pass through it. I don't know what is used to make camera optical components so I have no idea if they would work in this application. Just a word of caution on one of the less obvious possible issues.

Your other big cost will be suitable standard gases to calibrate the sensor with. Expect the response to be non-linear (see Beer-Lamberts Law), which means a number of gases will be needed for calibration purposes. WMO referenced standards are going to run you ~1k a pop. Standards from your local welding supply will be cheaper, maybe a few hundred. But labeled accuracy will be an order of magnitude or so worse than the sensor accuracy you are looking for.

Take the above to suggest that it is no easy task to build and calibrate an IRGA. My recommendation would be to look to an existing low cost sensor and then work towards modifying it to improve its performance.

[orcafood]

I'm taking the optical grade mirrors from the cameras, they should work well as the entry and exit mirrors they are probably close to a wavelength flat.

The concave collimating mirrors would be pricey your right. Perhaps less so out of old equipment though. Next question why even use the collminating mirrors? Why not just a tube with a silicone collimating lens (passes 1-10 um) at the end, no mirrors, just one lens which runs around $200. Of course the bandpass filter is still needed. Gas standards would be tricky to make. Could they not be made using a working CO2 meter? For example remove the CO2 from a gas space with soda lime and see how low it can go. That is the first point on the calibration. Add CO2 with a tank until it goes up 50 ppm on the working meter then set the point. 5-10 points would probably do?

Good information on the IR-lasers I was trying to find more about them, I was worried they would be cost prohibitive. (Off topic but holy cow I wish I had one of those Xe/Ar lasers, so much fun to use in the labs doing Raman. I never set it up, but I watched them do it for a little bit, takes foooreeevverrr)

A=-log(I/I0)=Elc brings me back, extinction coefficients are so useful. Absorbance and concentration seem linear in that equation or were you referring to how the linear range breaks down at the maximum linear response?

CO2 only has 4 vibration modes. This is its symmetric stretch, asymmetric stretch, and then the two scissors. Only 3 of these are IR active, the asymmetric stretch at 4.3 um and the degenerate scissors at 15 um which have a much weaker IR intensity. So it seems like the best choice is to focus on the 4.3 um area.

All that said I agree that a sensor would be much easier to setup and repeatable for all of us reefers. Please let me know if you can find a sensor with ppb resolution for a decent price, that would be a game changer! The $100 NDIRs seem to have a resolution around 20 ppm which is not good enough.

I couldn't find one so far that fit the bill, thus looking for alternatives.


Another thing I was reading about was that many of the HVAC NDIRs have a built in baseline correcting algorithm that assumes that the lowest value it can get to is around atmospheric CO2 levels. The detector drifts to lower CO2 concentrations so the baseline constantly gets changed under the assumption that indoor co2 will only be above atmospheric CO2 and never below. Not good when a headspace in contact with seawater may have less than atmospheric co2.